1. Field of the Invention
The invention relates to an apparatus and a process for the longitudinal-side welding of tubes to flat steel bars, which serve to produce gas-tight tube walls for industrial and power-station boiler plant.
2. Discussion of Background
To produce industrial and power-station boiler plants, thousands of meters of welded joints have to be made between the boiler wall tubes and flat steel bars arranged in between. These so-called tube walls form a large part of industrial or power-station boilers.
It is known that the submerged-arc welding (SAW) process in particular is used to weld such tube walls. The SAW process has the advantage that the arc burns in a stabilized manner and free of spatter under a powder layer, which at the same time assumes the function of providing protection against harmful atmosphere and deoxidation and of forming a slag for the satisfactory metallurgical course of the process. The process is distinguished by high deposition efficiency and a low frequency of defects. However, it has the disadvantage that welding can normally only be carried out in the so-called gravity position, i.e. in the horizontal plane.
This means that hitherto welding always had to be carried out in two passes during the welding of the boiler tube walls, for the quality requirements call for substantial through-welding between tube and flat steel bar. In this arrangement, the welding head is fixed and the parts to be joined are moved.
The joints are thus welded first of all on both sides from above in a first pass. Then the parts to be welded to one another are turned and finish-welded at the remaining two sides in a second pass. But this fact necessitates a relatively high proportion of handling and welding time in the entire fabrication process. Since the possibilities of increasing the welding speed have in the meantime been exhausted, only the use of another welding process is suitable for accelerating the entire process.
Overhead SAW processes (DE 26 40 270, DE 34 25 225, DE 34 30 349) are certainly also known, in which the weld pool, in comparison with "normal" SAW, has been turned as it were through 180.degree. and the welding powder and the electrode are supplied to the welding point from bottom to top. The welding powder has to be forcibly pressed there against the welding point. The main difficulty in overhead SAW consists in guaranteeing the formation of a sound weld, since the weld pool has to be held in suspension, which leads to the instability of the characteristics of the weld pool. The quality of the weld in this process is therefore often not as high as it ought to be according to the demands made on boiler plant. This is because welding with relatively high welding parameters is necessary here to achieve the specified penetration depth. A combination of the weld-pool and powder-supply devices is not possible at the given spatial relationships for the welding of the tube wall.
The gas metal-arc welding (GMAW) process has also been known for years for many applications, and it has been possible in recent years to make marked improvements to the process by means of electronics and through the use of new shielding gases. It is nowadays possible to synergetically control the transfer of droplets by pulsed arc, the result of which is that the weld pool can be readily controlled and modelled. Welding of the tube walls by GMAW therefore likewise appears possible. But this process has not gained acceptance hitherto for the welding of tube walls, since the SAW process is easier to manipulate for this purpose and, inter alia, additional protective measures, e.g. against the dazzling arc in the GMAW process, are not necessary in the SAW process.
To guide the welding torches in various welding processes, the use of oscillation units is known with which it is possible to widen the weld pool and thus bridge irregularities in the groove spacing and produce a weld of good quality. A disadvantage is that the conventional oscillation units on the market work relatively slowly.